In unison with the continuous advance toward higher integration of semiconductor devices, the photolithography process encounters an increasing demand for further miniaturization. With respect to the flatness of silica glass substrates for forming photomasks, even a photomask using a substrate having a satisfactory value of flatness has a likelihood that when mounted in an exposure tool by a vacuum chuck or holding means, the overall surface topography (or shape) of the photomask may be substantially deformed depending on the surface topography of portions of the substrate corresponding to the holding means of the exposure tool.
A photomask is generally prepared by depositing a light-shielding film on a silica glass substrate and patterning the film. Upon lithographic exposure, the photomask is often held horizontal by chucking the photomask surface at its outer peripheral portions by a vacuum chuck or holding means. The overall surface topography of the photomask may be substantially deformed depending on the surface topography of portions of the substrate corresponding to the holding means of the exposure tool. For this reason, attempts were made to select a substrate having minimal deformation and to bevel a substrate near chamfered surface portions so as to minimize deformation of a pattern-bearing central region of the substrate.
For instance, JP-A 2003-050458 discloses that a substrate is judged on a pass/fail basis by measuring the flatness of the substrate and simulating the shape of a photomask substrate after vacuum chucking. If more substrates fail, the manufacture process suffers from substantial wastes. JP-A 2004-029735 refers to the flatness of a substrate, but not to the chucking in an exposure tool, indicating that the photomask shows an insufficient flatness when mounted in the exposure tool.
WO 2004/083961 proposes a reticle substrate configured to minimize the deformation of the reticle vacuum chucked in an exposure tool wherein the shape of a substrate surface extending to the outermost periphery near the boundary of a chamfered surface is defined. The substrate must be inspected by a probe type topography measuring apparatus in order to confirm the shape outside the range that is measurable by a flatness tester of optical interference type. This means that the substrate after polishing is subject to contact inspection, with an increased risk of damages being caused by handling. Productivity is undesirably low due to an increased number of inspection steps.